This invention relates to a route searching system of a navigation apparatus used for finding the shortest route from a given point to a destination, and, in particular, to a system of preparing a node series leading to a connecting intersection when a starting point and a destination are set between intersections.
Navigation apparatus are known which guide a driver who is not familiar with a particular region to his destination in that region by selecting a suitable course leading thereto. Apparatus of this kind are now being developed eagerly.
In a conventional navigation apparatus, a course along which a driver will be led to his destination is set based upon a starting point and a destination point which are input before the driver starts traveling. The driver navigates along the set course. There are various systems of designating a desired course during navigation. In one such system a map is displayed on a CRT screen and an image of the course is superposed on the map. In another system information about an intersection where the driver needs to turn the vehicle in accordance with a predetermined course is displayed, which information includes numerals or a graph indicating the distance remaining until the intersection is reached and a picture showing certain features of the intersection. Another system makes use of speech output as an auxiliary means.
Ordinarily, in a road system, a plurality of courses which connect a starting point and a destination point could in theory be selected. For this reason, various methods of searching for a shortest-time or shortest-distance course (shortest course) between a desired starting point and destination point have been proposed for application to navigation apparatus. One example of such a method has been reported in which an intersection, such as that shown in FIG. 11, is represented by eight nodes and sixteen directional links to express left and right turns, straight-ahead travel and U-turns, and in which each of the roads connecting the intersections are represented by a pair of directional links. In another example (such as the one disclosed in Japanese Patent Laid-Open No. 62-91811), the shortest course is found by comparing data on courses along on which travel is prohibited, and if travel is prohibited, the course is removed thereby finding a shortest courses where includes no impassable section is to be found.
Generally speaking, when inputting starting points and destinations in such a navigataion apparatus, the points to be inputted are limited to intersections and some particular points which are specified. Accordingly, the driver must move the vehicle to such a specified point and then input the point as the starting point before he can be guided through navigation.
As described above, it is not possible with conventional navigation apparatus to set points other than intersections and some particular points as the starting point or destination. Consequently, a driver who is at a position which does not include such specified points needs to move to the nearest guidable point, such as an intersection, by himself without any guidance.